1. Field of the Invention
The present invention relates to a method of recycling a coated magnesium alloy product. More particularly, the present invention relates to a method of removing a coating from a coated magnesium alloy product such as a magnesium alloy casing of a notebook computer or a cellular phone, a method of making a recycled magnesium alloy, and a method of recycling a coating of a coated magnesium alloy product.
2. Description of the Related Art
The casings of electronic devices such as notebook computers, cellular phones, PDAs are required to be strong, to efficiently diffuse heat generated by the inside components and to be suitable for efficient recycling. To meet such requirements, instead of conventional resin casings, metal casings are increasingly used for such mobile electronic devices.
As materials for forming such metal casings for electronic devices, attention is focused on light alloys mainly composed of light metals such as magnesium (Mg) or aluminum (Al) for making light electronic devices. Particularly, Mg has the highest specific strength among simple metals which can be used as a structural material, has a high heat dissipation capability which is comparable to Al, and has a low specific gravity which is about 70% of that of Al. Therefore, Mg alloy mainly composed of Mg can be suitably used for making the casings of electronic devices.
For molding articles from Mg alloy, die casting or thixomolding may be utilized. When a casing of a notebook computer, for example, is formed from molten Mg alloy by die casting or thixomolding, about 30-50 volume % of the molten metal injected by a single time of injection finally becomes portions of the casing. The remaining about 50-70 volume % of the molten Mg alloy solidifies in a sprue or a runner of the mold and is later cut away. In this way, with the die casting or thixomolding, a large part of the material is wasted in each time of the molding. Therefore, to attain the cost reduction and good use of resources of the earth, there is a demand for a technique for recycling the portions of the Mg alloy product which have conventionally been wasted in the molding process and the Mg alloy obtained from collected products to reuse as a material for injection molding.
JP-A-2000-47257 discloses a method of making a recycled Mg alloy from the portions of the Mg alloy product cut away after the injection molding. With this method, the portions of the Mg alloy product cut away after the injection molding are remelted together with flux, and its composition is controlled in the molten state. Then, the material is cooled for solidification to provide a recycled Mg alloy having an intended composition.
Generally, commercially available Mg alloy parts or products have coated surfaces. Unfavorably, when such Mg alloy product formed with a coating is melted for recycling, a large amount of organic gas is generated due to the burning of acrylic resin or urethane resin contained in the paint. Further, the molten metal may be unacceptably contaminated due to titanium or the like contained in the paint. Therefore, for getting a recycled Mg alloy from wasted products, the coating on the surfaces of the Mg alloy product need be removed before melting the Mg alloy product.
Techniques for removing coatings from the surfaces of the Mg alloy product are disclosed, for example, in JP-A-2000-263443 and JP-A-2001-20018. Specifically, JP-A-2000-263443 discloses wet blasting, wherein the coating is physically removed by spraying water working as fluid with particles of inorganic substance such as alumina. With the wet blasting, however, the proper removal of the coating is difficult when the coated surface of the Mg alloy product is irregular including projections and recesses. Specifically, the recesses are unlikely to be hit by the inorganic particles so that the coating at the recesses may not be sufficiently removed. Conversely, the projections are likely to be excessively hit by the inorganic particles so that the Mg alloy may be removed together with the coating.
JP-A-2001-20018 discloses thermally treating the coated Mg alloy product at a temperature at which the paint vaporizes in non-oxidizing atmosphere. However, this method requires a chamber for performing the thermal treating under particular conditions. Moreover, since a large amount of organic gas is generated after the paint is decomposed, this method further requires an apparatus for properly treating such gas.
As an alternative known method, the coating may be removed from the surfaces of the Mg alloy product using alkaline solution. Specifically, with this method, the coating is removed by immersing the coated Mg alloy product in alkaline solution for swelling the coating due to the action of the alkaline solution followed by rinsing for example. Conventionally, in such a method, the immersing of the coated Mg alloy product is performed until the coating entirely swells. Specifically, since such a coating is generally made up of an undercoat layer and an overcoat layer, the coated Mg alloy product continues to be immersed in alkaline solution until the both layers swell, which may take a long time of about two hours. Further, when the coating swells, metal powder and the like contained in the coating dissolves or diffuses, contaminating the alkaline solution. Therefore, the alkaline solution used until the entirety of the coating swells tends to be degraded relatively early during its repetitive use for the immersion.
JP-A-5-84746 and JP-A-6-114842 also disclose a method for removing a coating. However, these methods are intended for removing coating from a molded resin product, not an Mg alloy product.
Moreover, conventionally, the coating removed from the Mg alloy product by the immersion and the following rinsing is treated as industrial waste. Such treatment of the coating is not desirable in view of the good use of natural resources and the environmental preservation.